Enhanced superconductivity in atomically thin 2H-TaS$_{\mathrm{2}}$ down to the ultra-thin limit

One attractive candidate for realizing superconductivity in the 2D limit is transitional metal dichalcogenides (TMDs). For example, studies of NbSe$_{\mathrm{2}}$ have verified superconductivity down to monolayer, with T$_{\mathrm{c}}$ reduced from 7 K (bulk) to 3 K (monolayer). Similar to NbSe$_{\mathrm{2}}$, 2H-TaS$_{\mathrm{2}}$ is another candidate, with very similar crystal structure yet T$_{\mathrm{c}}$ less than 1 K in bulk. Prior work attempting to access the 2D limit of 2H-TaS$_{\mathrm{2}}$ has found significant sample degradation after fabrication, and it turns to insulating behavior when thickness is less than 3.5 nm. We are able to overcome it by encapsulating TaS$_{\mathrm{2}}$ flake with hBN in a glove box while also using thin pre-evaporated metal contacts to maintain a good van der Waals `seal' to prevent air from reacting with the sample. With this method, we find that superconductivity persists down to the bilayer limit. More surprisingly, we observed pronounced increase in the T$_{\mathrm{c}}$ from around 0.7 K (bulk) to 3 K (bilayer) when the thickness of the flake is reduced, opposite to the trend observed in most superconducting films as well as recent studies on NbSe$_{\mathrm{2}}$. This provides interesting evidence that reducing the dimensionality can actually strengthen superconductivity as opposed to the weakening effect that has been reported in other 2D materials.